1. Field of the Invention
The present invention relates to a sawing suspension which is composed of an essentially anhydrous or nonaqueous organic liquid in which hard-material particles are dispersed. The sawing suspension is used in conjunction with a wire saw for cutting wafers from a crystal of brittle and hard material.
2. The Prior Art
Wire saws are used, for example, in order to divide silicon crystals into thin wafers. A sawing suspension, referred to as a slurry, which is transported from the saw wires to the crystal is used in this process as an agent with abrasive action. The sawing suspension is composed of hard-material particles which are dispersed in a liquid. Sawing suspensions with an oil base and with an aqueous base are already known.
From EP-686 684 A1 it is disclosed that it is disadvantageous to use an oil as a liquid for the sawing suspension. The reason for this is that the cleaning of oil-contaminated parts of the wire saw and the cleaning of the wafers are very expensive. A sawing suspension which has an aqueous base and which contains water- soluble polymers is therefore proposed.
According to JP-4,218,594, if an aqueous sawing suspension is used which contains certain glycols and thickening agents, the extent of the superficial damage to the wafers caused by the cutting operation is reduced.
U.S. Pat. No. 4,853,140 describes how the choice of an aqueous cooling lubricant favorably influences the shape (geometry) of silicon wafers. This occurs if the silicon wafers are cut from a crystal with a sawblade while using this cooling lubricant.
If a wire saw is used as the sawing tool, care also has to be taken to provide that the cut wafers have lateral surfaces which are as flat as possible. These lateral surfaces, in addition, should also be parallel to one another. Deviations from this ideal shape are typically quantified by measuring the thickness variation (TTV value) and the sag (warp value) of a wafer.